In recent years, a plurality of TDMA (Time Division Multiple Access) wireless LAN systems have been proposed and standardized in the 2.4 GHz or 5 GHz ISM (Industrial, Science, and Medical) bands. In the TDMA scheme, communication is carried out by allocating different time regions for individual mobile stations in the same frequency band. In the TDMA wireless LAN systems, there are a centralized control scheme and a distributed control scheme.
FIG. 12 shows the configuration of a wireless LAN system of the centralized control scheme. As shown in FIG. 12, a wireless LAN system 101 of the centralized control scheme comprises a base station 103 for allocating communication time bands (time regions for carrying out communications) to mobile stations, and a plurality of mobile stations, for example a first, a second and a third mobile station 105, 107 and 111, whose timings of communication are controlled by the base station 103.
The base station 103 has a central control device 115, for example, either built inside or connected thereto. It may also be connected to a wide area network (WAN) 117. The base station 103 and the first to third mobile stations 105, 107 and 111 can communicate with one anotherviasignals 118-1 to 118-3.
FIG. 13 shows an example of the configuration of a second wireless communication system 102 of the distributed control scheme. As shown in FIG. 13, the second wireless LAN system 102 comprises a first to a fourth mobile station 121, 123, 125 and 127. The first to fourth mobile stations 121, 123, 125 and 127 can communicate with one another via signals 131-1 to 131-4. The mobile stations issue communication requests to one another and adjust the timing of communication between them individually.
Representative of the wireless communication system of the centralized control scheme are ARIB (Association of Radio Industries and Businesses) STD-T70 (HiSWANa), ETSI (European Telecommunications Standards Institute) HiperLAN/2, and Bluetooth developed by an industry association, for example.
Representative of the distributed control scheme are IEEE802 committee 802.11a, IEEE 802 committee 802.11b, and ARIB RCR STD-T33, for example.
When communications are performed using wireless communication devices according to the aforementioned standards, or using notebook PCs (personal computers), PDAs (personal digital assistants) or the like equipped with such devices, it is necessary to avoid mutual interference that occurs when communication areas overlap upon one another.
In the aforementioned 802.11b or Bluetooth, for example, a frequency hopping scheme is defined (whereby the transmission channel is changed at certain time intervals to limit the time of occupation of the transmission channel and thereby reduce the influence of mutual interference.).
Further, various communication control methods have been proposed or standardized, including the CSMA/CA scheme such as 802.11a, 802.11b or STD-T71 (in which some collision preventing function is provided to prevent the collision (interference) of signal radio waves, in addition to carrier sensing, such that if the channel is in use for example, carrier sensing is continued until the channel is available).
However, in reality, the above-mentioned interference avoiding methods only help to reduce interference.
JP Patent Publication (Kokai) No. 2001-45027 discloses a technique relating to the avoidance or reduction of interference between different systems. The publication discloses the base station of a first wireless communication system transmits a transmission permission signal to the base station of a second wireless communication system by polling. In response to the transmission permission signal, the base station of the second wireless communication system transmits transmission permission signal delay information to the base station of the first wireless communication system. The base station of the first wireless communication system then decodes the transmission permission signal delay information and delays transmission permission to the mobile station included in the first wireless communication system. The base station of the first wireless communication system and that of the second wireless communication system communicate to one another, so that a band can be ensured for use by the second wireless communication system, which, using the band, can carry out wireless communications.
By using this technique, the second wireless communication system signal can be transmitted by polling by which communication is permitted by designating a communication time in the occupied frequency band of the first wireless communication system, so that the same frequency band can be shared between different systems.
However, the technique described in the above publication is difficult to be applied to the case where at least one of the wireless communication systems to be made to coexist is of the centralized control scheme in which there is a periodic frame configuration and the operation is based on a control signal transmitted by a base station.
The present invention provides a technique for preventing interference among a plurality of wireless communication systems including one of the centralized control scheme so that bands can be efficiently managed.